Testing apparatus



May 3, 1932.

L'. T. wlLsoN TESTING APPARATUS Filed Jun.10, 1931 Brwe ATTORNEYPatented May 3, 1932 UNITED STATES PATENT OFFICE LEON T. WILSON, OFCHATHAM, NEW JERSEY, ASSIGNOR T AMERICAN TELEPHONE AND TELEGBAPHCOMPANY, A CORPORATION OF NEW YORK TESTING APPARATUS Application ledJune 10, 1931.

rIhis invention relates to testing apparatus and more particularly toarrangements for measuring leakage condnctances.

Tn studying the properties of transmission lines it is desirable toaccurately measure the leakage conductance introduced into the lines bythe insulators employed. From the standpoint of leakage the insulator isequivalent electrically to a leakage conductance shunted by a capacity.

One of the early methods of measuring the leakage conductance of theinsulator involved connecting the insulator or group of insulators to bemeasured in` one arm ot a bridge so that in that arm there would be theequiv- 2".; sistance in the second arm ot the bridge and shunting theconductance and capacity of the element to be measured by a variablecapacity in the Vhrst arm of the bridge. By this arran gement theconductance of the element to be measured would be the only realcomponent of the impedance of the first arm ot' the bridge and this realcomponent would be balanced by the single adjustable resistance whichconstituted the only real component of the impedance in the second armot the bridge. Therefore, when the condition of balance was obtained,the conductance of the element to be measured could be computed in termsof only two variables, namely, the variable resistance and thefrequency.

Serial No. 543,445.

Later on a method of balancing was devised in which the two arms of thebridge were nor- Inally provided with equal resistances. The resistancein the first arm of the bridge was connected in shunt with the unknownconductance and capacity of the element to be measured and theresistance in the second arm was connected in shunt with a capacity. Abalance of the real components of the impedance was then effected byadjusting the resistance element in one of the arms. With thisarrangement the leakage was for the condition of balance a function ofthe normal value of the two resistances, the adjust-ed value of the oneresistance, and the amount of adjustment. This setup had two advantages:(a) the conductance to be measured was under the conditions ol balanceindependent of the frequency and was, furthermore, such a function ot`the several resistance values above mentioned that over a certain smallrange the conductance was approximately directly proportional to theamount oi adjustment of the adjustable resistance and could, therefore,be read directly from the adjustment.

However, with this improved method it was found that the conductance tobe measured could only be read directly from the amount of adjustment ofthe adjustable resistance over a relatively small part of the totalrange oi conductance involved in carrier transmission. Accordingly, bythe present invention an improved method of measurement has beenprovided in Which the conductance to be measured may be read in terms ofthe adjustment of a resistance element over very much wider range ofvalues than was possible with the former method. In accordance with thepresent invention the bridge, as before, is provided with two normallyequal resistances in the two arms, one of these resistances beingconnected in shunt with the unknown conductance, and capacity of theelement to be measured and the other resistance being connected in shuntWith the capacity.

The bridge, in accordance with the new method, is balanced by varyingthe two resistances by equal increments in opposite directions or, inother words, by increasing the one resistance and decreasing the other.When the condition -of balance is reached the conductance will be afunction of the amount of adjustment of the two resistances and of theadjusted values of the two resistances. Since the departure from alinear relation between the value of the conductance and the amount ofthe adjustment for the one adjustable resistance is opposite to thatresulting from an adjustment of the other resistance, the departuresfrom linearity in the two cases tend to cancel each other so that thelinear relation holds over a much wider range of values, and in practiceit has been found that this range of values may be made to cover allvalues of conductance ordinarily encountered in the entire carrierfrequency range over which the measurements are to be made.

The invention will now be more fully understood from the followingdescription, when read in connection with the accompanying drawings,Figures l, 2, 3 and 4 of which show in simple schematic form differentmethods of balancing a bridge in order to make measurements of theconductance of insulators, and Fig. 5 is a similar diagram sh-owing amethod of balancing the bridge in accordance with the present invention.

TheI present invention, while applicable to the measurement of leakageconductances of any kind, is of particular importance in connection withtests of insulators such as are employed in transmission lines. Forexample, it has been common practice to set up a number of shorttransmission lines each involving a limited number of pairs ofinsulators upon cross-arms. Usually each short line to be measuredinvolves about twentyfive pairs of insulators. Each insulator introducesa certain amount of leakage loss from the one line conductor to theother. If this combination of elements be viewed from one end of theline the arrangement is electrically equivalent to a leakag-econductance shunted by a capacity, the combination being bridged acrossthe two terminals at the end of the line. If, now, these two terminalsof the line be connected in one arm of a bridge, the real and imaginaryparts of the impedance as seen from the bridge may be balanced by avariable resistance in series with a variable capacity connected in abalancing arm of the bridge.

. Such an arrangement is shown in Fig. l in which the insulator orcombination of insulators whose conductance is measured is representedby a conductance Gin shunt with a capacity C1, these elements beingbalanced by the adjustable resistance R in series with the adjustablecapacity C in the second arm of the bridge. The bridge, of course, maybe of any balanced type such as a Wheatstone bridge, the two sets ofelements above described being connected in two arms of the bridge. Ithas been found, however, that the hybrid coil type of bridge is anadvantageous form of bridge to use, and accordingly, this type of bridgeis illustrated in Fig. l and comprises a balanced transformer or hybridcoil l0 across the midpoint of which alternating current source S isconnected, a receiver R being inductively related to the hybrid coil asshown. The two balancing arms are then connected to the oppositetermina-ls of the-line windings of the hybrid coil.

With this set-up it is evident that the impedance Z of the bridge armcomprising the variable resistance R and the variable capacity C will berepresented by the following equation:

1 Z -R jwO (l) in which j is the operator 1/- 1 and w is Qa times thefrequency. This equation may be rewritten in the following form:

1 yw z-iitoi (2) Now by multiplying both numerator and denominator byRjwCLI-l, this expression becomes l R202 Zriwom-mawom .(3)

From Equation (3) it will be seen that the admittance (the reciprocal ofthe impedance) of the left-hand arm of the bridge in Fig. l has twocomponents, a real component and an imaginary component. ln order that aperfect balance may be obtained these two components must each beindividually balanced by corresponding real and imaginary components ofthe admittance of the righthand arm of the bridge. rl`he real part ofthe admittance of the right-hand arm of the bridge is, of course, theunknown conductance G to be measured. Setting this equal to the realpart of Equation (3) we have as the condition for balance. In actualpractice the factor R2w2C2 is small compared with unity so that Equation(Il) becomes G=Rw202 aproximately. (5)

From Equation (5) it will be seen that when the bridge is balanced theconductance will be a function of the adjusted value of the resistanceR, the adjusted value of the capacity C, and of the frequency factor w.Therefore, in order to compute the value of the conductance G after thebalance had been obtained R, C, and o had to be known. Furthermore,since the conductance G is a function of the squares of C and o thesetwo factors had to be known accurately a small error in either gave amagnified error in the conductance.

The calculations :for the various values of G involved in a large numberof measurements were very laborious when the method of balancing shownin Fig. l was employed. The matter was simplified somewhat, however, byemploying the method of balance indicated in Eig. 2. Here the capacity Cin the left-liand arm of the bridge is liXed and the adjustment forbalancing the imaginary parts of the impedances of the two arms wasobtained by adjusting a variable capacity Cl bridged across theright-hand arm of the bridge in shunt with the unknown capacity C1 ot'the element to be measured. Under the condition of balance G would be afunction of R, o, and C, as indicated in Equation (5) but since C wasiiXed it was only necessary to malte the computations in terms of thetwo variables R and o.

)Vinile this reduced the labor ot computing the values of G to someextent, it was 'found that the method of balancing shown in Fig. 3 wouldfurther reduce the laborof computation. In F ig. 3 the conductance G andthe capacity Cl of the unknown element to be measured was shunted by aresistance X. Instead of having a. resistance and capacity in series inthe balancing arm ot' the bridge as in Fig. 2, a variable capacity C isconnected in shunt with a variable resistance X-E in the balancing arm.Normally the variable resistance in the balancing arm has a value Xequal to the corresponding resistance in the right-hand arm. In order toobtain a balance for both the real and imaginary components ot theinipedances ot' the two arms, the capacity C is adjusted and theresistance X is reduced by successive steps until a balance is obtained.For the condition ot` balance the resistance in the lefthand arm ot' thebridge has been reduced from a value X to a value X-R- The real part ofthe impedance of the right-hand arm is now due to the unknownconductance G in parallel with the resistance X while the real part ofthe impedance ot' the right-hand arm will be X-R. If the real parts ofthese two impedances are now equated we have:

Consequently, under conditions of balance G is a direct function of R,the amount ot adjust-ment of the lefthand resistance, and an inversefunction oi' the normal value of the left-hand resistance, and theadjusted value oi' the left-hand resistance. From Equation (6) it willbe apparent that this method oi' balancing has the advantage that G canbe computed without taking into consideration the frequency. This methodof balancing the bridge also has a further advantage Jfor it will benoted trom Equation (6) that when R is small, G is also small, andconsequently, for small adjustments of the resistance in the left-handarm the followingl approximation is quito accurate:

G=l (approximately) (7) From Equation (7) it is evident that X beingconstant G is thus directly proportioned to R for small adjustments ofthe resistance in the rightshand arm of the bridge. However, thisproportionality does not hold over much of the range of values of Gencountered in tests of insulators to be used for carrier transmissionand so the exact relation represented by Equation (6) had to be used inthe computation of values of G for most oi the range. Even so the methodot' balancing represented in Fig. 3 is quite use-ful and thecalculations of G were much less laborious on the whole than for themethods of Figs. l and 2. A very important feature oit' the method ofbalancing shown in Fig. 3 is the elimination of both C, the iiXedresistance, and o the frequenction function, from the expression for G.

An arrangement similar to Fig. 3 is shown in E ig. 4C. In this instance,however, the resistance X in the left-hand arm ofthe bridge remainsfixed and the resistance in the righthand arm oi' the bridge isincreased from a normal value X by successive increments until a balanceis obtained. Under conditions of balance the resistance in the righthand arm of the bridge will have a value .Xi-R, or, in other words, willhave been increased by an amount It. The imaginary components ot' theimpedances ot' the two arms of the bridge may be brought into equalityas in 3 by adjusting the capacity C in the left-hand arm of the bridgeor, it desired, the capacity C may remain Iixed and an adjustablecapacity shunted across the right-hand arm of the bridge may be variedas was done in the case of Eig. 2. In either event the adjustment of thecapacity does not enter into the computation for the values of theconductance G.

Since the set-up ofFig. 4L is essentially similar to that of Fig. 3,under conditions of balance we have @moms 8) and, again, for smallvalues of G,

G=2i (approximately) (9) It will be noted by comparing Equation (8) withEquation (6) that the departure les rit

DI A

ali

from the linear relation in the case of Fig. 4 is in the oppositedirection from what it was in the case of Fig.- 3. This observationsuggests the possibility that by combining these two methods thedeparture from the linear relation might be kept small over a wide rangeof G. This combination is illustrated in Fig. 5. Here the tworesistances in the right-hand and left-hand arms of the bridge are asbefore made normally equal with a value X. In order to adjust the bridgethe resistance in the right-hand arm is increased by steps and theresistance in the left-hand arm is decreased by equal increments until abalance is obtained, in which case the right-hand resistance will havebeen increased to X-l-R and the left-hand resistance will have beendecreased to X-R. As before, the imaginary components of the impedancesof the two arms may be adjusted by varying a capacity in either therighthand or left-hand arm of the bridge.

When a balance is obtained the real component of the impedance of theleft-hand is X-R and the real component of the impedance of theright-hand arm is equal to the conductance G in parallel with theconductance of the resistance X-i-R. Therefore G will have the valueance is small, G may be represented as follows:

G ZX@ (approximately) (1 1) In this case for the linear relation to holdit is only necessary that R2 be small compared with X2 while in themethods illustrated in Figs. 3 and 1 it was necessary that R be small ascompared with X. Thus it is apparent that the arrangement of Fig. 5makes a departure a second order one, and therefore, it should permit awide range of G to be measured by use of the approximate expression ofEquation (11) with good accuracy. If this is s-o then G is directlyproportional to R and the measuring equipment can be arranged to readleakage conductance G directly or, better still, the leakage conductanceper pair of insulators may be read directly by suitable calibration ofthe adjustable steps of either adjustable resistance.

As already stated, in measuring the conductance of insulators it hasbeen the practice to arrange the insulators in pairs on Vcross-arms withwires strung along the cross-arms. The cross-arms for test purposes maybe located close to each other instead of being separated by distancesof the order of hundreds of feet as in actual pole line construction.Usually a group of twenty-ive pairs of insulators is measured.Experience with, tests of groups of insulators of this character showsthat one micro-mbo of conductance per pair of insulators is about ashigh a value as it is necessaryV to cover. For twenty-live pairs themaximum value of G then becomes 25 micro-mhos. For direct reading it isconvenient to choose R=1000 ohms when the conductance G per pair is i..

equal to 1 micro-mbo. On this basis since Nominal True reading leakage Gper G per Per cent pair pair error mcromicronihos mhos From this tableit is evident that values of leakage up to about .9 micro-mho per paircan be measured without the error due to the approximation exceeding 1per cent. As 95 per cent or more of the measurements have been found tofall in this range, the apparatus can thus be made direct reading fornearly all measurements. p

Where the apparatus is calibrated for direct measurements in casesinvolving twenty-five pairs of insulators, correction must, oi course,be made for the actual number of pairs where a lesser or greater numberof pairs than twenty-ive is to be measured.

For voice measurements .1 micro-mbo per pair is as high as is generallyrequired. Thus using the same variable resistance R to cover this rangeX2 becomes 800 (10)6 and X=28,28-1.3 ohms. Tn this range the linearrelation holds very accurately; in fact, better than l of one per centover most of the range.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated, without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. The method of measuring the conductance of an unknown element whichconsists in connecting the unknown in one arm of a bridge having abalancing arm, connecting normally equal resistanccs in shunt with theunknown and in the balancing arm respectively, and increasing the oneresistance and decreasing the other by equal amounts until a balance isobtained, whereupon the conductance to be measured will be approximatelya direct function of the change in resistance necessary to produce abalance.

2. The method of measuring the conductance of an unknown element whichis equivalent to a conductance shunted by a reactance, which methodconsists in connecting the unknown in one arm oi a bridge having areactance in its balancing arm, connecting normally equal resistances inshunt with the unknown and with said second-n'ientioned reactance,respectively, and increasing the one resistance and decreasing the otherby equal amounts until a balance is obtained, whereupon the conductanceto be measured will be approximately a direct function ofthe change inresistance necessary to produce a balance.

3. The method of measuring the conductance of an unknown element whichis equivalent to a conductance shunted by a reactance, which methodconsists in connecting the unknown in one arm of a bridge having areactance in its balancing arm, connecting normally equal resistances inshunt with the unknown and with said second-mentioned reactancerespectively, and increasing the resistance in shunt with the unknownand decreasing the other resistance by equal amounts until a balance isobtained, whereupon the conductance to be measured will be approximatelya direct function of the change in resistance necessary to produce abalance. y

4t. The method of measuring the conductance of an unknown element whichis equivalent to a conductance Gr shunted by a capacity, which methodconsists in connecting the unknown in one arm of a bridge having a.capacity in its balancing arm, connecting normally equal resistances Xin shunt with the unknown and with said second-men.- tioned capacity,respectively, and increasing the one resistance and decreasing the otherby equal amounts R until a balance is obtained, whereupon G 2Rapproximately.

5. The method of measuring the conductance oi an unknown element whichis equivalent to a conductance G shunted by a capacity, which methodconsists in connecting the unknown in one arm of a bridge having acapacity in its balancing arm, connecting normally equal resistances Xin shunt with the unknown and `with said second-mentioned capacity,respectively, and increasing the resistance in shunt with the unknown.and de,- creasing the other resistance by equal amounts R until abalance is obtained, whereupon 2R @WT-7a over any range of values of G,and

approximately over a wide range of G.

6. In a bridge arrangement for measuring the conductance of an unknownelement, said bridge including arms having normally equal adjustableresistances and having said unknown element connected in shunt with theresistance in one arm, the method of measuring the conductance of theunknown which consists in increasing one of said resistances anddecreasing the other by equal amounts until the real components of theimpedances of the arms are balanced, whereupon the conductance to bemeasured will be approximately a direct function of the change inresistance necessary to produce a balance. i

7. In a bridge arrangement for measuring the conductance of an unknownelement which is equivalent to a conductance shunted by a reactance,said bridge including arms having normally equal adjustable resistancesandhaving said unknown element connected in shunt with the resistance inone arm and a. reactance in shunt with the resistance in the other arm,the method of measuring the conductance of the unknown, which consistsin increasing one of said resistances and decreasing the other by equalamounts until the real components of the impedances of the arms arebalanced, whereupon the conductance to be measured will be approximatelya direct function of the change in resistance necessary to produce abalance.

8. In a bridge arrangement for measuring the conductance of an unknownelement which is equivalent to a conductance shunted by a reactance,said bridge including arms having normally equal adjustable resistancesand having said unknown element connected in shunt with the resistancein one arm and a reactance in shunt with the resistance in the otherarm, the method of measuring the conductance of the unknown, whichconsists in adjusting the reactance in one of the arms until theimaginary parts of the impedances of the arms are balanced and inincreasing one of said resistances and decreasing the other by equalamounts until the real componente of the impedances of the arms arebalanced, whereupon the conductance to be measured will be approximatelya direct function of the change in resistance necessary to produce abalance.

9. In a bridge arrangement for measuring the conductance of an `unknownelement which is equivalent to a conductance shunted by a reactance,said bridge including arms having normally equal adjustable resistancesand having said unknown element connected in shunt with the resistancein the other arm,

the method of measuring the conductance of 5 the unknown, which consistsin increasing the resistance in shunt with the unknown and decreasingthe other resistance by equal amounts until the real components of theimpedances of the arms are balanced, whereupon the conductance to bemeasured will be approximately a direct function of the change inresistance necessary to produce a balance.

l0. In abridge arrangement for measuring the conductance of an unknownelement which is equivalent to a conductance shunted by a reactance,said bridge including arms having normally equal adjustable resistancesand having said unknown element connected in shunt with the resistanceinone arm and a reactance'in shunt with the resistance in t ie other arm,the method of measuring the conductance of the unknown, which consistsin adjusting the reactance in one of the arms until the imaginary partsof the impedances of the arms are balanced, and in increasing theresistance in shuntwith the unknown and decreasing the other resistanceby equal amounts until the real components of the impedances of the armsare balanced, whereu upon the conductance to be measured will beapproximately a direct function of the change in resistance necessary toproduce a balance. l

1l. In a bridge arrangement for measuri" ing the conductance of anunknown element which is equivalentV to a conductance G shunted by acapacity, said bridge including arms having normally equal adjustablere'- sistances X and having said unknown ele-Y ment connected in shuntwith the resistance in one arm and a capacity in shunt with theresistance in the other arm,'the method of measuring` the conductance Gof the unknown, which consists in increasing one of said resistances anddecreasing the other by equal amounts R until the real components of theimpedances of the arms are balanced, whereupon v 'G 2R X-zapproximately.

12. In a bridge arrangement for measuring the conductance of an unknownelement which is equivalent to a conductance G shunted by a capacity,said bridge including arms having normally equal adjustable resistancesX and having said unknown ele ment connected in shunt with theresistance in one arm and a capacityV in shunt with the resist-ance inthe other arm, the method of measuring the conductanceV Gr of theunknown, which consists in adjusting the capacity in one of the armsuntil the ini-Y are balanced and in increasing one of said aginary partsof the impedances of the armsifs'saaba rsistances amiaeefeaang uit therby equal amounts It until the real components of lthe impedances of thearms are balanced', where-` upon G :x2 approximately. l t 13. In abridge arrangement for measure ingthe conductance of an unknown elementwhich is kequivalent to a, conductance G shunted by a capacity, saidbridge including arms having normally equal adjustable ref sistances Xand having said unknown ele-v 2R FX2-R2V over any range of values ofGand 2R i #xi approximately over a wide range offG.

14. In a bridge arrangement for measur-- ing the conductance of anunknown element which is equivalent to a 'conductance G shunted by acapacity, said bridge including arms having normally equal adjustableVresistances X and having said unknown element connected shunt with theresistance in one arm and a capacity in shunt with the resistance in theother arm, the method of measuring the conductance'Gr of the unknown,which consists in adjusting the ca pacity in one of the arms until theimaginary parts of the impedances of the arms are balanced and inincreasing the resistance X in shunt with the unknown and decreasing theother resistance by equal amounts R until the real components of theimpedances of th arms are balanced, whereupon over any range of valuesof Gand y shunt with the unknown and said resistances being capable ofadjustment to'in'creasethe one and decrease the other by equal amountsuntil a balance is obtained, whereupon the conductance to be measuredwill be approxi-v mately a direct function of the change in resistancenecessary to produce a balance.

16. A bridge for measuring the conductance of an unknown element whichis equivalent to a conductance shunted by a reactance, said bridgehaving an arm in which the unknown may be connected and a balancing armincluding a reactance normally equal resistances in the arms connectedin shunt with the unknown and with said second-mentioned capacity,respectively, said resistances being capable of adjustment to increasethe one and decrease the other by equal amounts until a balance isobtained, whereupon the conductance to be measured will be approximatelya direct function of the change in resistance necessary to produce abalance.

17. A bridge for measuring the conductance of an unknown element whichis equivalent to a conductance shunted by a reactance, said bridgehaving an arm in which the unknown may be connected and a balancing armincluding a reactance, normally equal resistances in the arms connectedin shunt with 'the unknown and with said second-mentioned reactance,respectively, said resistances being capable of adjustment to increasethe resistance in shunt with the unknown and decrease the otherresistance by equal amounts until a balance is obtained, whereupon theconductance to be measured will be approximately a direct function ofthe change in resistance necessary to produce a balance.

18. A bridge for measuring; the conductance of an unknown element whichis equivalent to a conductance Gr shunted by a capacity., said bridgehaving an arm in which the unknown may be connected and having abalancing arm including a capacity normally equal resistances Xconnected in the arms in shunt with the unknown and with saidsecond-mentioned capacity, respectively, said resistances being capableof adjustment so that the one resistance may be increased and the otherdecreased by equal amounts R until a balance is obtained, whereupon thevalue of the conductance Gr may be read directly from the amount ofadjustment of the resistance in accordance with the relationapproximately.

19. A bridge for measuring the conductance of an unknown element whichis equivalent to a conductance G shunted by a capacity, said bridgehaving an arm in which the unknown may be connected and having abalancing arm including a capacity, normally equal resistances Xconnected in the arms in shunt with the unknown and with saidsecond-mentioned capacity, respectively, said approximately.

In testimony whereof, I have signed my name to this specilication this8th day of June 1931.

LEON T. WILSON.

